There was a nice thread on GS on a close question. AFAIR, with XV5050/5080 + SRX-07, you'll get all the same samples - but not the specific presets for JD-990 that are there on SR-JV80-04. Another point to consider is the filter resonance + headroom issue inherent to JDs.

You may also try asking Don Solaris directly, he's an expert in JD-990.